a) Field of the Invention
This invention relates to an air bag system, which is used in a state mounted in a central part of a steering wheel or in an instrument panel to protect a driver or an occupant in a front passenger's seat in the event of a collision accident.
b) Description of the Related Art
In the event of a collision accident, the so-called primary collision that an automotive vehicle collides with another automotive vehicle or the like is followed by occurrence of the so-called secondary collision that an occupant such as the driver hits the steering wheel. With a view to reducing an impact exerted on the occupant's body at the time of the secondary collision and hence protecting the occupant, air bag systems are finding increasing utility.
An air bag system promptly inflates a bag in the event of a collision accident and supports the body of an occupant by the inflated bag. Known air bag systems include, for example, those disclosed in Japanese Patent Applications Laid-Open Nos. HEI 1-109146, 4-27639, 4-55147, 5-131886 and 5-162601 and Japanese Utility Model Applications Laid-Open Nos. HEI 4-54747, 4-115968 and 5-12306. FIG. 12 of the accompanying drawings shows an air bag system whose basic structure is disclosed in Japanese Patent Application Laid-Open No. HEI 1-109146.
An air bag module 1, which constitutes the air bag system, is constructed of a housing 2 fixed on an unillustrated steering wheel (in the case of an air bag system for a driver's seat), an inflator 3 for blowing out high-pressure gas such as nitrogen gas upon occurrence of a collision accident, said inflator 3 being supported and fixed in a central part of the housing, an air bag 4 for being inflated by the high-pressure gas in the event of the collision accident, said air bag 4 being supported in a folded form on the housing 2, and a cover 5 normally extending over the air bag 4.
Of these elements, the air bag 4 defines an opening 7 having an inner diameter which is large enough to permit insertion of a gas blow-out portion 6 of the inflator 3. Around the gas blow-out portion 6 of the inflator 3, a peripheral edge portion of the opening 7 lies on the housing 2 and is held between the housing 2 and an annular holding plate 8. By bolts 9 and nuts 10, the holding plate 8 is fixed on the housing 2.
The cover 5 has been formed by injection molding a synthetic resin or the like and has a cover plate portion 11 extending over the air bag 4 and a mounting plate portion 12 disposed on one side of the cover plate portion 11. The housing 2, on the other hand, has been formed by drawing a metal plate and has a base plate portion 13 for supporting the inflator 3 and the air bag 4 thereon and a support plate portion 14 formed on a peripheral edge portion of the base plate portion 13 to connect and fix the mounting plate portion 12.
To connect and fix the cover 5 to and on the housing 2, the mounting plate portion 12 is held between an outer side wall of the support plate portion 14 and a holding strip 15. These three superposed members 12,14,15 are connected together by blind rivets 18 or by connecting members such as bolts and nuts. A flange portion 16 is formed on and along an end edge of the mounting plate portion 12. End edges of the flange portion 16 are maintained in engagement with proximal end edges of the support plate portion 14 and the holding strip 15, respectively, so that the mounting plate portion 12 is prevented from slipping down between the support plate portion 14 and the holding strip 15.
Upon occurrence of a collision accident, the inflator 3 blows out high-pressure gas responsive to a signal from an unillustrated impact detection sensor so that the air bag 4 is promptly inflated. At this time, the cover 5 is torn at a thin wall portion 17, thereby allowing the air bag 4 to pop out of the cover 5. As a result, the air bag 4 is allowed to inflate on a rear side of the steering wheel and supports the driver's body.
In the air bag system which is constructed and operable as described above, the structure of a portion where the mounting plate portion 12 of the cover 5 is connected to and fixed on the support plate portion 14 of the housing is preferred to meet the following conditions (1) to (3):
(1) Inclusion of positioning means for limiting the positional relationship between the support plate portion 14 and the mounting plate portion 12. PA1 (2) Inclusion of displacement-preventing means between the support plate portion 14 and the mounting plate portion so that both the plate portions 14,12 are prevented by the displacement-preventing means itself from displacement in the direction of planes in which the plate portions lie, respectively. PA1 (3) Need for fewer parts for the connecting and fixing portion between the support plate portion 14 and the mounting plate portion 12.
The support plate portion 14 and the mounting plate portion 12 are connected and fixed together by registering circular holes, which are formed in both the plate portions 14,12, with each other and inserting connecting members such as bolts and nuts or rivets in the circular holes. Without the positioning means described above, it is not easy to have the circular holes registered with each other as described above, thereby making the assembly work of the air bag system cumbersome.
In the construction shown in FIG. 12, for example, the mounting plate portion 12 of the cover 5 is strongly pulled upwards as viewed in FIG. 12 upon actuation of the air bag system (i.e., upon inflation of the air bag 4 based on blow-out of high-pressure gas from the inflator 3). To ensure actuation of the air bag system, the connection between both the plate portions 14 and 12 themselves must be surely prevented from separation by the pulling force.
Unless the displacement-preventing means is present, it becomes necessary to increase the connecting force for both the plate portions 14,12 by using more connecting members. Use of more connecting members undesirously leads to the need for more assembling steps.
If many parts are needed, the fabrication, management and assembly of the parts becomes cumbersome. The need for such many parts is therefore not preferred.
The construction shown in FIG. 12, that is, disclosed in Japanese Patent Application Laid-Open (Kokai) NO. HEI 1-109146 can meet the conditions (1) and (2) out of the above-described conditions (1) to (3) but cannot meet the condition (3). Namely, the holding strip 15 is indispensable to surely keep the flange portion 16, which is formed on the mounting plate portion 12, in engagement with the end edge of the support plate portion 14 so that the connecting strength between the support plate portion 14 and the mounting plate portion 12 can be retained.
If this holding strip 15 is omitted, the engagement between the flange portion 16 and the end edge of the support plate portion 14 becomes insecure so that upon actuation of the air bag system, a large force may be applied to the mounting plate portion 12 at areas where the blind rivets 18 have been inserted.